The present invention relates to a zoom lens system and, more particularly, to a compact and high-performance, two-group type of zoom lens system making use of a plastic lens, which is best fit for use on a lens shutter camera having a zoom ratio of about 2.
For achieving an inexpensive and compact zoom lens, it is of course effective to reduce the number of lenses used. Reducing the number of lens groups used should be preferable as well, because this makes it possible to simplify the construction of the lens frame, reduce performance degradation due to eccentric displacements of the groups and attain stable performance at low costs. In such a context, reducing the number of lenses used in a two-group type of zoom lens system is effective for achieving a compact and inexpensive zoom lens.
Even in the case of a zoom lens for a compact camera having a zoom ratio of 2 or thereabout, however, it is imperatively required to use an aspherical surface for gaining good-enough performance, if five or fewer lenses are used to this end.
One typical example of the prior art aiming at making a zoom lens compact by reducing the number of lenses used is set forth in Japanese Provisional Patent Publication No. 3-127008. However, the example of this publication teaches the use of an aspherical glass surface. Of course, the use of a glass lens places some limitation on cutting down the price of a zoom lens; to put it another way, the use of a plastic lens is inevitable for reducing that price further. For instance, this is typically disclosed in Japanese Provisional Patent Publication No. 2-181110.
When a plastic lens is employed, however, attention should be paid to large changes in its refractive index and lens configuration due to temperature and humidity changes. The above-mentioned publication states that any focal position displacement occurring by temperature changes can be set off by the use of positive and negative lenses. This procedure may be effective only in the context that the refractive index changes due to temperature changes are stabilized within a relatively short span of time. However, it is indeed impossible to neglect surface configuration changes which are affected by how lenses are held by the lens frame, and so it is very difficult to predict focal position displacements, if occurs.
When there are humidity changes, however, much time is needed for stabilizing lenses' refractive indices and configurations. If the power of a single plastic lens is strong, as in the above-mentioned example of the prior art, the focal position will indeed be subject to some large displacement in the course of humidity absorption, although depending on to what degree each lens absorbs humidity. In addition, when varying plastic materials are used at the same time, as in the above-mentioned example, even larger focal position displacements will occur due to the fact that the degree of humidity absorption differs from time to time.